1. get current log on business user’s uuid via Context.GetCurrentIdentityUUID()
2. get the identification instance of the found business user by query
3. get the corresponding employee instance of current business user
4. get assigned organization unit of found employee got from step 3
5. get the department name of found organization unit from association NameAndAddress
6. create a transient field in a custom BO and write the above logic into AfterLoading event. The script file for this AfterLoading is marked as NOT mass-enabled.

import ABSL;
import AP.PC.IdentityManagement.Global;
import AP.FO.BusinessPartner.Global;

var queryByIdentityUUID = Identity.QueryByElements;
var queryByIdentityUUIDParameter = queryByIdentityUUID.CreateSelectionParams();
var queryByEmployeeBPUUID = Employee.QueryByIdentification;
var queryByEmployeeBPUUIDParameter = queryByEmployeeBPUUID.CreateSelectionParams();


if ( this.DepartmentName.IsInitial()){

	var id = Context.GetCurrentIdentityUUID().content;
	queryByIdentityUUIDParameter.Add( queryByIdentityUUID.UUID.content, "I", "EQ", id.ToString() );
	var result = queryByIdentityUUID.Execute(queryByIdentityUUIDParameter);
	var first = result.GetFirst(); // points to identity instance
	var person = first.Person;
	var bpUUId = person.UUID.content;
	queryByEmployeeBPUUIDParameter.Add( queryByEmployeeBPUUID.UUID.content, "I", "EQ", bpUUId.ToString());
	var employeeQueryResult = queryByEmployeeBPUUID.Execute(queryByEmployeeBPUUIDParameter);
	var EmployeeQueryResultCurrent = employeeQueryResult.GetFirst();
	var assignedOrg = EmployeeQueryResultCurrent.OrganisationalUnitAssignment.GetFirst();
	var org = assignedOrg.ToRoot;
	  // readOnly in AfterLoading event
	   this.DepartmentName  = org.NameAndAddress.AddressSnapshot.NameSuitableForLogonLanguage.GetFirst().Name.SecondLineName;
	}